1. Field of the Invention
The invention relates to a method for determining a frequency/time profile of hits, and a device for carrying out the method.
In particular, the method and the corresponding device are used in the analysis of the measurement signal of a distributed optical temperature sensor.
A device for carrying out the method according to the preamble of claim 1 is sold by the firm LeCroy under the name "1879 PIPELINE MULTIHIT TIME-TO-DIGITAL CONVERTER" (see LeCroy Corporation, Research System Division: "1992 Research Instrumentation Catalog", page 51-53).
2. Discussion of Background
In distributed optical temperature sensors, a short laser pulse is injected into a multi-mode fiber. From the intensity of the backscattered Raman light as a function of time after the laser pulse, the local temperature distribution in the fiber can be inferred. Because the Raman light is very weak, a large number of such single measurements (measurement cycles) must be averaged.
In addition to analog methods, the so-called photon counting is also suitable for the time-dependent detection of the Raman light. In this method, the time of arrival of a single backscattered photon in a preselected time window (measurement window) is usually determined. Using such a single-photon method, a very good time resolution is achieved (and thus also a good spatial resolution in the temperature measurement. However, because only one photon from the backscattered volume of light is utilized in a measuring cycle and because the processing of the hit still takes a certain time after the time window has elapsed, the measuring time necessary for good averaging of the signal is very long. The measuring time can be shortened by not only a single but the largest possible number of photon hits being processed within one time window.
This requirement is met, for example, in the above-mentioned multihit time-to-digital converter which is sold by LeCroy. In the text which follows, a hit is understood to be the rising or falling edge of a digital signal. Photons can be converted into such signals in a special detection circuit.
The central element of the 1879 PIPELINE MULTIHIT TIME-TO-DIGITAL CONVERTER is a fast shift register. During a measuring cycle, the shift register is filled in such a manner that its content, a series of digital "1" and "0", corresponds to the occurrence in time of the hits to be measured. In the shift register, the time interval, measured from the start of the measuring cycle to the occurrence of a hit, is converted into a binary number. A stop signal ends the measuring cycle. Subsequently, a decision can be made during a pause between measurements where a hit is to be taken into consideration or not. After this pause between measurements, the hits are time coded and read into a memory from where they can be subsequently read out for final processing.
In the measuring instrument mentioned, hit detection, hit analysis and final hit processing take place in succession. First all hits are detected. Only then are all hits analyzed and provided for final processing. The measuring time is long and is 412 .mu.s+approximately 50 ns per hit in this instrument. A new measuring cycle can only be started after the pause between measurements has elapsed. The final processing, i.e. the determination of the frequency/time profile, does not occur during the measurement but only thereafter.